1. Field of the Invention
The present invention relates to a photosensitive composition, a lithographic printing plate precursor, and to a lithographic printing method using same. More particularly, it relates to a lithographic printing plate precursor which permits direct plate making by scanning a laser light having a wavelength of, for example, from 300 to 1,200 nm based on digital signals from a computer, called a lithographic printing plate precursor for a direct plate-making process, novel cyanine dyes which can be preferably used therefor, and a lithographic printing process of directly developing the lithographic printing plate precursor on a printing machine without a development processing step and performing printing.
2. Description of the Related Art
A lithographic printing plate generally comprises oleophilic image areas which accept an ink in a printing step and hydrophilic non-image areas which receive dampening water. Lithographic printing is a printing method of producing areas different in ink adhesion properties on the lithographic printing plate, with the oleophilic image areas thereof being ink-receptive areas and hydrophilic areas thereof being dampening water-receptive areas (ink-repelling areas), utilizing the properties of water and oil to repel each other, inking only the image areas, and then transferring the ink onto a material to be printed such as paper.
In order to prepare this lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image recording layer) has conventionally been widely used. Ordinarily, this lithographic printing plate precursor is exposed through an original of a lith film, and image areas of the image recording layer is allowed to remain, whereas non-image areas of the image recording layer is dissolved away with an alkaline developing solution or an organic solvent to thereby reveal the surface of the hydrophilic support and make a printing plate. Thus, there is obtained a lithographic printing plate.
In the conventional plate-making process using a lithographic printing plate precursor, a step of removing, after exposure, non-image areas of the image recording layer by using a developing solution selected according to the kind of the image recording layer has been required. Thus, it has been one subject to eliminate the necessity of, or to simplify, such additionally performed wet treatment. In particular, in recent years, disposal of the waste liquor discharged accompanying the wet treatment has been a great matter of concern throughout the field of industry in view of the consideration for global environment, and hence solution of the above-described subject has been increased more and more.
As one of simple plate-making methods in response to the above-described requirement, there has been proposed a method, called on-machine development, wherein a lithographic printing plate precursor having an image recording layer the non-image areas of which are capable of being removed in a conventional printing process is used and, after exposure, the non-image areas of the image recording layer is removed on a printing machine to obtain a lithographic printing plate.
Specific methods of the on-machine development include, for example, a method of using a lithographic printing plate precursor having an image recording layer that can be dissolved or dispersed in dampening water, an ink solvent or an emulsion of dampening water and ink, a method of mechanically removing an image recording layer by contact with rollers or a blanket cylinder of a printing machine, and a method of lowering cohesion of an image recording layer or adhesion between an image recording layer and a support upon penetration of dampening water, an ink solvent or the like and then mechanically removing the image recording layer by contact with rollers or a blanket cylinder of a printing machine.
Additionally, in the invention, unless otherwise indicated particularly, the term “development processing step” means a step of using an apparatus (ordinarily, an automatic developing machine) other than a printing machine and removing infrared laser-unexposed areas of the image recording layer of the lithographic printing plate precursor upon contact with liquid (ordinarily, an alkaline developing solution) thereby revealing a hydrophilic surface of support. The term “on-machine development” means a method and a step of removing infrared laser-unexposed areas of the image recording layer of the lithographic printing plate precursor upon contact with liquid (ordinarily, printing ink and/or dampening water) by using a printing machine thereby revealing a hydrophilic surface of support.
On the other hand, digitization technique of electronically processing, accumulating and outputting image information using a computer has been popularized in recent years, and various new image outputting systems responding to the digitization technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate (CTP) technique of carrying digitized image information on highly converging radiation, for example, laser light and performing scanning exposure of a lithographic printing plate precursor to the light thereby directly preparing a lithographic printing plate without using a lith film. Thus, it is one of important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
As is described above, in recent years, the simplification of plate-making operation and the realization of dry system and non-processing system have been more strongly required than ever from both aspects of the consideration for global environment and the adaptation for digitization.
Of the lithographic printing plate precursors, a lithographic printing plate precursor having on a hydrophilic support an oleophilic photosensitive resin layer containing a photosensitive compound capable of generating an active species such as a radical or a Brφnsted acid by laser exposure has been proposed as a lithographic printing plate precursor capable of being scan exposed and has already been marketed. A negative-working lithographic printing plate can be obtained by scan exposing this lithographic printing plate precursor to a laser light based on digital information to generate an active species which functions to cause physical or chemical change in die photosensitive layer to insolubilize the layer, and then development processing the exposed precursor. In particular, a lithographic printing plate precursor comprising a hydrophilic support having provided thereon a photo-polymerization type photosensitive layer with an excellent photosensitive speed containing a photo polymerization initiator, an addition-polymerizable ethylenically unsaturated compound, and a binder polymer soluble in an alkali developing solution and, as needed, an oxygen barrier protective layer can form a printing plate having desirable printing performance due to the advantages that the precursor is excellent in productivity, permits simple development processing, and shows good resolution and ink-depositing properties.
In addition, as a lithographic printing plate precursor which can be on-machine developed, a lithographic printing plate comprising a hydrophilic support an image-forming layer wherein hydrophobic thermoplastic polymer particles are dispersed in a hydrophilic binder is described in Japanese Patent No. 2,938,397. Japanese Patent No. 2,938,397 describes that it is possible that, after exposing the lithographic printing plate precursor by an infrared laser to coalesce the hydrophobic thermoplastic polymer particles by heat and to form an image, the precursor is mounted on a cylinder of a printing machine and is subjected to on-machine development with dampening water and/or an ink.
Although this method of forming an image by coalescing fine particles through mere thermal fusing shows good on-machine developability, it involves the problem that the image strength is so weak that the resulting printing plate has insufficient printing durability.
In addition, JP-A-2001-277740 and JP-A-2001-277742 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) describe a lithographic printing plate precursor comprising a hydrophilic support having provided thereon an image recording layer (heat-sensitive layer) containing microcapsules encapsulating a polymerizable compound.
Further, JP-A-2002-287334 describes a lithographic printing plate precursor comprising a support having provided thereon an image recording layer (photosensitive layer) containing an infrared ray absorbent, a radical polymerization initiator, and a polymerizable compound.
These methods of employing polymerization reaction are characterized in that, in comparison with image areas formed by thermally fusing polymer fine particles, formed images have comparatively good image strength owing to high chemical bond density in the image areas. In view of practical viewpoint, however, on-machine developability, fine line reproducibility, and printing durability are all still insufficient.
Still further, US 2003/0064318 describes an on-machine developable lithographic printing plate precursor comprising a support having provided thereon an image recording layer containing a polymerizable compound and a graft polymer having polyethylene oxide chain as side chain or containing a block polymer having ethylene oxide blocks.
However, this technique provides insufficient fine line reproducibility and printing durability, though on-machine developability thereof is good.
In general, an operation for inspection and discrimination of image formed on a printing plate is carried out in order to examine whether the image is recorded on the printing plate as intended, in advance of mounting the printing plate on a printing machine. With a conventional lithographic printing plate precursor subjected to the development processing step, it is ordinarily easily performed to confirm the image formed after the plate-making (after the development processing) and before the printing (before mounting the printing plate on a printing machine) by means of coloration of the image recording layer.
However, with respect to the lithographic printing plate precursor of the on-machine development type or non-processing (non-development) type without accompanying the development processing prior to printing, the image is not recognized on the printing plate in the step of mounting it on a printing machine, and thus the discrimination of the printing plate can not be performed. In particular, it is important in the printing operation to determine whether a register mark which acts as a landmark for the registering in multicolor printing is recorded. Therefore, with the lithographic printing plate precursor of the on-machine development type or non-processing (non-development) type, a means for confirming the image at the stage of exposure or heating is required, that is, exposed or heated areas are required to form or lose color.
Printing plates have been proposed wherein a compound capable of generating an acid, base or radical by means of light or heat and a compound capable of undergoing color change upon interaction with the acid, base or radical generated are used as the print-out agents (for example, see JP-A-11-277927). Also, it has been proposed to utilize color change of thermally decomposable compound as the print-out agent of a direct-drawing type lithographic printing plate precursor having a heat-sensitive layer (for example, see JP-A-2000-335129). Further, it has been proposed to use a thermally decomposable dye having a decomposable temperature of 250° C. or below as the print-out agent (for example, see JP-A-2003-191657).
According to these techniques, color is formed or lost in exposed areas, which has served to improve viewability (plate-inspecting properties) of an image to some extent. However, the improvement is still practically insufficient and, in addition, there have been involved various problems such as a problem that formed dyes or dyes not decomposed by exposure and losing color in turn dye the dampening water to exert adverse effects on finished prints, a problem that insoluble dyes remain in an ink or dampening water, and a problem that components of an ink or dampening water react with the dye to form a precipitate, i.e., tailings.